1. Field of the Invention
The present invention relates to a layer 2 switch (hereinafter referred to as L2SW) which terminates a layer 2 frame (hereinafter referred to as Ethernet frame) and, more particularly, to improvement in a method of processing an expansion VLAN tag applied to an Ethernet frame for expansion.
2. Description of the Related Art
Expansion VLAN technique is a technique developed for employing VLAN techniques used in a conventional L2SW to individually separate users in a wide area Ethernet for the use as VPN. With a local area network divided into segments, VLAN has been originally used as a means for shutting up an Ethernet broadcast frame or a means for ensuring security. In order to meet a demand from communication service providers for making use of the VLAN techniques for VPN for the purpose of individually separating users accommodated in a network in wide area Ethernet service, communication apparatus manufactures have developed the techniques on their own. In addition, while IP-VPN techniques have been conventionally used, wide area Ethernet service is attracting more and more attention because of apparatus introduction costs, operation costs, facility of introduction and the degree of freedom to make the communication service providers consider introduction of various kinds of services using the VLAN techniques.
In wide area networks provided by communication service providers, one communication service provider's network is divided into a plurality of small areas.
This arrangement is made, in order to avoid a problem that an expansion VLAN is capable of accommodating 4096 users at the maximum because the expansion VLAN employs the same tag format as that of a common VLAN, for the purpose of ensuring expandability in a provider's network as a whole by dividing one area into small units to limit the number of users accommodated in each area.
As shown in FIG. 8, the entire network is structured to have Ethernet networks each set up in each area connected with each other through an L2SW. Each area is an independent LAN in which as many as 4096 VLAN can be set. When transmitting and receiving data to bridge the areas, once delete an expansion VLAN tag at an exit of each area and add, at an entrance of a next area, an expansion VLAN tag inherent to the area.
In addition, as shown in FIG. 9, another method is considering each area as a layer to sequentially add/delete an expansion VLAN tag when moving through the layers. In this case, the higher a layer goes up in hierarchy, the more are applied the expansion VLAN tags.
One of conventional art similar to the present invention is that recited in Kohyo (National Publication of Translated Version) No. 2001-500345.
With a system shown in FIG. 9, because an expansion VLAN tag is inserted every time a layer goes up in hierarchy, an overhead in an Ethernet frame (other part than data) is increased. This results in having a compressed band in the network to adversely affect L2SW processing performance.
In addition, since many of communication service providers set the maximum frame length allowed by an L2SW in a network to be 1522 or 1526 bytes, setting of all the L2SW in the network should be changed. Furthermore, because old type devices fail to cope with such an expansion VLAN system, setting change is not enough to be adapted to the system, so that replacement of the apparatus will be required.
With the system shown in FIG. 8, it is necessary to arrange L2SW for gate way at each of an entrance and an exit of each area to connect the areas.
As a result, the number of L2SWs is increased in the entire network to invite an increase in operation costs of the apparatus.